This invention relates to data communications systems, and, more particularly, to a system and method for improving the system efficiency of a data communication system. An upstream communication channel connecting a central control unit and a plurality of terminals is time-shared with the terminals by controlling the timing of every terminal""s data transmission according to the operation states of the terminals in the system.
An interactive data communication system using an existing cable television (CATV) network has recently provided fast communication services for members. The data communication system primarily comprises a central control unit and a plurality of members terminals, communicating with each other using a space frequency band on the CATV network organized with hybrid fiber coaxial (HFC) cables.
The data communication system has a communication channel consisting of a set of opposite channels, namely, a downstream channel and an upstream channel. A packet, which controls the timing of each terminal""s data transmission (hereinafter, referred to as a xe2x80x9cMAPxe2x80x9d)is carried via the downstream channel from the central control unit to each terminal. On the other hand, a packet, which requests transmission of data (hereinafter, referred to as a xe2x80x9crequest packetxe2x80x9d), and a packet which includes data itself (hereinafter, referred to as xe2x80x9cdata packetxe2x80x9d) are carried via the upstream channel in the opposite direction. The upstream channel is time-shared among the terminals. A minimum unit of the time-divided upstream channel is called a xe2x80x9cmini-slotxe2x80x9d. The central control unit controls assignments of mini-slots on the upstream channel through the MAP.
The central control unit broadcasts a MAP informing every terminal which mini-slots accept request packets. A terminal which desires to transmit data to the central control unit sends a request packet for the indicated, available mini-slot. After collecting these request packets for a predetermined period of time (hereinafter, referred to as xe2x80x9clatencyxe2x80x9d), the central control unit assigns a desired number of mini-slots for transmission of the data to each corresponding terminal and broadcasts a subsequent MAP informing each terminal of the assignment of the mini-slots. Finally, each corresponding terminal transmits the data to the central control unit according to the assignment.
A theoretical distance between the central control unit and a terminal causes a transmission delay which comprises (1) a downstream propagation delay to allow the terminal to receive a MAP, (2) a processing time of the terminal to allow the terminal to parse and respond to the MAP, and (3) an upstream propagation delay to allow the central control unit to receive a data packet. Therefore, the central control unit examines every terminal""s transmission delay and reserves the maximum transmission delay caused by the most theoretically distant terminal. And the central control unit decides the proper latency in consideration of the maximum transmission delay.
However, all terminals are not always in operation in the data communications system. Consequently, the system may waste time. Furthermore, when a new terminal is added to the system or an existing terminal is removed from the system, the predetermined latency may be improper.
Accordingly, it is an object of the present invention to provide a system and method for improving system efficiency of the data communications system. The method may thus control the timing of every terminal""s data transmission according to the operation state of the terminals in the systems.
A data communications system, consistent with the present invention, comprises a central control unit, a plurality of terminals, and a communication channel connecting the central control unit and the terminals. The communication channel includes upstream and downstream channels. The upstream channel is time-shared with the terminals and each of the time-shared channels is assigned to one of the terminals requesting one of the time-shared channel by the central control unit. The central control unit includes a measuring means, a determining means, and a controlling means. The measuring means measures the transmission delay between the central control unit and each of the terminals via the communication channel. The determining means determines the maximum transmission delay out of the transmission delays. The controlling means controls the timing of the data transmission in the system on the basis of the maximum transmission delay.